Components of many electronic, electromechanical, optical or other devices need to be assembled with precise alignment to assure optimal performance. In the case of certain magnetic recording disk drives, for example, the read/write head needs to be carefully positioned during disk usage with respect to the surface of the disk to assure optimum performance and to avoid crashing into the disk and causing damage.
Magnetic recording hard disk drives that utilize a head assembly for reading and/or writing data on a rotatable magnetic disk are well known in the art. In such systems, the head assembly is typically attached to an actuator arm by a head suspension assembly comprising a head suspension and an aerodynamically designed slider onto which a read/write head is provided so that the head assembly can be positioned very close to the disk surface. Such a head position during usage, that is, where the head is positioned over a spinning disk, is defined by balancing a lift force caused by an air bearing that spins with the disk acting upon the aerodynamically designed slider and an opposite bias force of the head suspension. As such, the slider and head “fly” over the spinning disk at precisely determined heights.
Head suspensions generally include an elongated load beam with a gimbal flexure located at a distal end of the load beam and a base plate or other mounting means as a proximal end of the load beam. According to a typical two piece head suspension construction the gimbal flexure comprises a platform or tongue suspended by spring or gimbal arms. The slider is mounted to the tongue thereby forming a head suspension assembly. The slider includes a read/write magnetic transducer provided on the slider and the slider is aerodynamically shaped to use the air bearing generated by a spinning disk to produce a lift force. During operation of such a disk drive, the gimbal arms permit the slider to pitch and roll about a load dimple or load point of the load beam, thereby allowing the slider to follow the disk surface even as such may fluctuate.
The head slider is precisely mounted to the flexure or slider mounting tongue of a head suspension at a specific orientation so as to fly at a predetermined relationship to the plane of the disk surface. During manufacturing and assembling of the head suspension assembly, any lack of precision in forming or assembling the individual elements can contribute to a deviation in the desired relationship of the surfaces of the elements. A buildup of such deviations from tolerance limits and other parameters in the individual elements can cause a buildup of deviation from the desired relationship of the head slider to the associated disk surface in the complete head suspension assembly. The parameters of static roll attitude and static pitch attitude in the head suspension assembly generally result from these inherent manufacturing and assembly tolerance buildups.
Ideally, for optimum operation of the disk drive as a whole, during assembly of the head slider to the slider mounting tongue, the plane of the load beam mounting surface datum and the plane of a head slider surface datum should be in a predetermined relationship to each other. The load beam mounting surface datum and the slider surface datum are usually planar surfaces that are used as reference points or surfaces in establishing the relationship of the plane of the actuator mounting surface and the plane of the surface of the head slider surface relative to each other. The upper and lower planar surfaces of the head slider are also manufactured according to specifications usually requiring them to be essentially or nominally parallel to each other.
Another critical performance-related criteria of a suspension is specified in terms of its resonance characteristics. In order for the head slider to be accurately positioned with respect to a desired track on the magnetic disk, the head suspension should be capable of precisely translating or transferring the motion of the positioning actuator arm to the slider. An inherent property of moving mechanical systems, however, is their tendency to bend and twist in a number of different modes when subject to movements or vibrations at certain rates known as resonant frequencies. At any such resonant frequencies that may be experienced during disk drive usage, the movement of a distal tip of the head suspension assembly, or its gain, is preferably minimized by the construction of the head suspension assembly. Any bending or twisting of a head suspension can cause the position of the head slider to deviate from its intended position with respect to the desired track, particularly at such resonant frequencies. Since the disks and head suspension assemblies are driven at high rates of speed in high performance disk drives, the resonant frequencies of a head suspension should be as high as possible. Resonance characteristics are usually controlled by precision construction, design and manufacture of the load beam. Accordingly, any changes or deformation to a head suspension after it is constructed, such as may be done for adjusting the static attitude of a head suspension assembly component may adversely affect the resonant characteristics of the head suspension assembly. Prior art static attitude adjusting techniques, such as described in U.S. Pat. Nos. 5,832,764 and 5,682,780, suffer from this disadvantage in that they teach modifying the shape or bending head suspension components in ways that adversely affect resonant characteristics. For example, U.S. Pat. No. 5,832,764 teaches modifying the spring region of the load beam (which region creates the bias force) that is found to be critical in controlling resonant characteristics.
Static attitude angles of a head suspension are commonly measured while the head suspension or head suspension assembly is clamped or fixtured in a loaded state so as to simulate its flying position. That is, a loaded state is created with the base plate rigidly secured and the load beam loaded (urged against its bias force), usually by a pin near its center, to be positioned at its intended fly height. Generally, such loading is performed on the load beam because it is very difficult to directly load a slider mounting tongue or a mounted slider thereto without affecting its static attitude angles. However, loading of the load beam itself is also difficult because of the clamping and fixturing that is needed. Such load beam loading can also introduce an angular bias because the loading force is not applied at the slider mounting tongue or slider. Additionally, non-centered loading of the load beam may further introduce an angular bias.
In practice, several optical methods may be used to measure the angle of component surfaces, such as laser triangulation of interferometry. Another such optical method is known as autocollimation. An autocollimator is able to measure small surface angles with very high sensitivity. Light is passed through a lens where it is collimated prior to exiting the instrument. The collimated light is then directed toward a surface, the angle of which is to be determined. After being reflected by the surface to be measured, light enters the autocollimator and is focused by the lens. Angular deviation of the surface from normal to the collimated light will cause the returned light to be laterally displaced with respect to a measurement device such as an eyepiece or a position sensing device. This lateral displacement is generally proportional to the angle of the surface and the focal length of the lens. An advantage of such a device is that the angle measurement is independent of the working distance of the lens or the distance between the instrument and the component being measured.
For some applications, white light sources are used with autocollimators. The light is directed through a pinhole to create a point source at a distance from the lens equal to the focal length of the lens. The position sensing device and the light source generally need to be at the same distance from the lens in order to obtain high resolution of the readings. Because the source and detector cannot physically occupy the same space, a beam splitter is usually utilized to mechanically offset the light source and position sensing device from one another. Typically, a 90-degree beam splitter is used.
Laser light sources are also frequently utilized for autocollimators. A main advantage is that the high intensity of the laser beam creates ultra-low noise measurements, increasing the accuracy and repeatability of the measurements. The high laser intensity also increases the working distance and permits angle measurement from non-mirror-like surfaces. Finally, the high laser intensity allows smaller spot sizes which enable measurement from small surfaces. A further advantage of a laser source is that incident white light will generally not interfere with the measurement, because the position sensing device can be chosen to be sensitive to the particular laser wavelength used. This approach typically focuses the laser to a point at a distance equal to the lens focal length in order to maintain high resolution of a position sensing diode. A disadvantage of this approach is that the surface of the lens is never perfect and some light can be reflected back onto the position sensing diode. That is, when the instrument is used to measure poorly reflective surfaces, the reflected light from the back side of the objective lens can be at approximately the same intensity as the light being reflected from the surface to be measured. As such, an accurate measurement is very difficult, if not impossible.
Additionally, in prior art devices the laser spot exits the device having the size and shape of the laser source, which can typically be greater than the workpiece surface to be measured. Thus, if the surface to be measured is generally smaller than the size and shape of the laser spot, an external mask may be needed to reduce the size of the spot on the component surface. An external mask adds mechanical positioning complexity and decreases the light in the return path, resulting in generally lower intensity of light at the detector.
Prior art equipment for determining and adjusting static attitude requires that individual suspensions be loaded into a tooling fixture for precisely aligning a component thereof to an autocollimator beam while bending the component to a desired position. This measurement takes a considerable amount of time and requires significant operator handling. It also requires that a head suspension loading mechanism, such as discussed above to simulate flying, consistently deform the head suspension component without damaging the component. Further complications include small positional misalignments between the autocollimator beam and the component to be measured. Generally, such misalignments can lead to erroneous measurements. A still further complication with common autocollimator based static attitude measurements lies with the fact that the autocollimator beam is masked very close to the measured component. The mask serves to only allow a certain desired location to be measured on the head suspension component. This masking technique can interfere with other mechanisms desired to operate in and around the component, blocks a portion of the light trying to return to the autocollimator, and obstructs the visual view of the component.
While numerous mechanisms exist to measure and adjust suspensions for static attitude, several limitations exist. A first limitation exists with those methods that act on the load beam as described above because of the possibility of introducing undesirable static attitude angle bias. Also, adjustment to the load beam can cause an undesired shift in load beam dominant resonant frequencies and gains. Additionally, equipment for acting on the load beam can be generally complex and expensive. Accordingly, there remains a continuing need for improved head suspension determining and adjustment equipment and methods. In particular, there is a need for equipment and methods for determining and adjusting head suspension and head suspension assembly static attitude.